Metal film attenuator



Sept 14, 1954 E. WEBER ETAL 2,689,294

' METAL FILM ATTENUATOR original Filed June 14, 1944 2 Sheets-sheet 1 En?. l

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sept. 14, 1954 METAL FILM ATTENUATOR 2 Sheets-Sheet 2 Original Filed June 14, 1944 Srmentors l M w. g. www mild-Entente!! Patented Sept. 14, 1954 S ATENT GFFICE 2,689,294 METAL FILM ATTENUATOR UITED sur Original application June 14, 1944, Serial No. 540,347. Divided and this application January 16, 1948, Serial No. 2,705

12 Claims. 1

This invention relates to-attenuators for electric waves of high frequency. While the devices of the present invention are especially useful in the ultra high frequency range covering frequencies from 300 to 10,000 and more megacycles per 5 very high frequencies, the electromagnetic eld second, the principles of the invention may be surrounding the conductors, or conducted within applied to other frequency ranges. the single hollow conductor, penetrates into the This is a division of our co-pending application metal only very slightly. It can .be shown that SerialvNo. 540,347, filed June 14, 1944 (Patent No. the electromagnetic field, or the current density, 2,529,436). Y decreases exponentially from the surface of the An object of the invention is to produce thin conductor towards the interior of the conductor, metal films vwhich are highly stable with respect the active part of the conductor being limited to humidity, temperature, and other infiuences to a depth of about three times the depth of and which may be used as conductors of high penetration of the current at the frequency of frequency currents. In the present invention the transmission, and any additional depth or thickfilms are used in attenuator structures for atness of conductor has very little effect upon the tenuating high frequency currents, but the films active resistance of the transmission line. The may be employed for other purposes. For expresent invention utilizes this characteristic of ample, they may be employed as resistance elehigh frequency transmission to secure the desired ments in current measuring systems. attenuation of power by including in the trans- According to the present invention, the metal mission conductor a section having a relatively films formed on insulating supports have a thickthin thickness with respect to the depth of penness which is thin by comparison with the depth etration. of penetration of the alternating current such Attenuation of power flow in a transmission that the direct current resistance becomes an 25 line can be accomplished by placing power abeffective measure of the high frequency resistance sorbing materials into the strongest magnetic of the film. field of at least one of the conductors of the line, Another object of the invention is to devise or by placing the material in the electrostatic highly stable metal films having a low temperafield existing between two conductors of a multure coefficient of resistance. The films are tiple conductor line. In the first case, the atformed of the dried and baked residue of a metaltenuator would be a series element which forms a lic solution, lpreferably a mixture of solutions of linear section of one of the conductors or wave two or more pure noble metals. Such films have guide, and in the second case the attenuator a temperature coefficient of resistance as low as would be formed as a shunt unit placed between 1A; to 1A of the coefficients of the component 35 the two conductors of a transmission line or armetals. Furthermore, the films have higher ranged transversely of a wave guide. specific resistivity than' the component metals, Heretofore, powdered materials of semi-con- Which provides a practical advantage in producductive or conductive nature have been used as tion since it permits thicker films than for pure power absorbing or attenuator units, but such metals. 40 materials have the disadvantage of being un- VA further object of the inventionds to devise stable at high temperatures and are subject to a unitary structure embodying an attenuator variation with frequency, humidity, temperature, element which may be inserted in a standard coand even with the density of the current or elecaxial line as a fixed element or With a structure tromagnetic field. According to the present inpermitting a variable degree of attenuation. vention the attenuator units are formed of stable Still another object is to devise fixed or variable metal films carried by a dielectric carrier or supattenuators for use in wave guides. port, the films having a thickness of only a frac- The attenuators of the present invention protion of the depth of penetration for the frequency vide substantially constant rate of power absorpof transmission and having a low temperature tion especially for moderate attenuation values coefficient. and for all frequencies belowV frequencies for Inacoaxial transmission system, the series type which the thickness of the attenuator lm is a of attenuator may be inserted in either the center small fraction of the depth of penetration. conductor or the outer conductor, but it is pre- For the transmission of power atany frequency, ferred to embody the attenuator in the inner and vat low transmissionV loss, the transmission conductor to prevent radiation from the attenualine may be formed of two conductors in any desired mutual arrangement, or the power may be transmitted by one conductor in the form of a hollow tube commonly called a wave guide. At

3 tor unit. Where the unit is embodied in the outer conductor of a coaxial system, or in a hollow wave guide, the unit must usually be properly shielded to prevent excessive radiation.

The invention .and various embodiments thereof will be described in connection with the accompanying drawing in which:

Figure 1 is a curve showing variation in current density from the surface of a conductor towards the interior thereof;

Figure 2 is a longitudinal sectional View of one form of attenuator assembly for a coaxial line with the attenuator unit in the center conductor;

Figures 3 and 4 are impedance diagrams for the series and shunt types of attenuatorsrespectively;

Figure 5 is a longitudinal sectional view of one end of an attenuator unit showing a preferred form of terminal connector;

Figure 6 shows a series attenuator unit embodied in theouter conductor of a coaxial cable or in a wave guide;

Figure 7 is a longitudinal sectional view showing one form of shunt attenuator unit in a coaxial c able;

Figure 8 isa transverse sectional view of a coaxial cable showing four dierent forms of shunt Yattenuator units;

Figure 9 is a sectional View of Figure 8 taken along lines 9 9; and

Figure 10 is a transverse sectional view of a rectangular wave guide showing a shunt attenuator unit mounted therein.

Figure 1 of the drawing shows the manner in which the current density in a conductor carrying high frequency current decreases from the surface of the conductor towards the interior thereof. As shown, with the current density at the surface of the conductor represented as 100 the current decreasesaccording to an exponential curve. .At a ldistance from the surface of the conductor the current has decreased to 36.8% of the density at the surface, the distance being the depth of penetration of the current at the frequency of transmission. The depth of penetration is -dened vas the'depth at which the current density is yequal to .l/e of the current density at the surface, .e being the base of the natural logarithm. At 36 the current density has dropped to substantially 51% which may be said to be the practical limit of .current penetration, and .any thickness of conductor beyond this amount has very little veiect on the resistance of the conductor. The value of is determined by the nature of the conducting material and the frequency .of transmission. The yfactors are given in Equation 41 in an article eby Ernst Weber appearing on pages 103 vto 112 of Electrical Engineering for March, 1943. The metal films used in the present Ainvention .have a Athickness .less than the depth of penetration, and preferably only a fraction of this amount. I-n the ultra high frequency range, the lms'may have a thickness of the order of 100 angstrom units, although the thickness may be varied and will-depend upon specic requirements of any given installation.

Figure 2 is shown a .unitary `attenuator assembly which may be inserted .in a -coaxial line. This assembly comprises a :short tubular housing I which preferably has the same diameter .and wall thickness as the .outer conductor 4of the coaxial line into which the assembly is to be .inserted. A short section .2 of the center conductor is mounted by suitable means in one end of tube I., .and the outer end of this conductor section is provided with a socket 2a which receives the plug end of the cented conductor from the adjacent section of the transmission line shown in dotted lines at 2h. The conductor section 2 may be :mounted in tube -I by any suitable means, but for the purpose of illustration it is shown in Figure 2 as being supported by a metallic stem 3 supported in a lateral extension 3a of tube I, the electrically eiective length of the stem 3 being lequal to l/4 of the wave length of the current being transmitted.

The attenuator ,unit is embodied in the center conductor and is formed of an insulating rod or `tube .4 (preferably Pyrex glass or quartz) having a thin metallic nlm formed on the outer surface of the rod or tube. The tube 4 has an external diameter equal to the outside diameter of the center conductor of the coaxial system and is preferably provided with bullet type connector terminals 5 at opposite ends. The attenuator unit .is supported in :the assembly by Venga-gement of one of the bullet terminals -in a socket formed in .the inner :end of conductor `section 2, while the other bulletter-minal extends out of -tube I by a distance suitable tor entering `the socket `end of the center conductor `of the adjacent section of the coaxial cable. It will be understood that when the attenuator assembly is inserted in the coaxial cable., the assembly is securely joined .to the adjacent sections of the coaxial cable by suitable coupling devices involving the threaded nuts B, -6 which have threaded engagement with coupling sleeves (not shown.) which maintain tube I in alignment with the -outer conductorsections of theeoaxial cable.

The tattenuator unit is Ipreferably formed in the following manner: .A suitable length of tubing .4 is vcoated on theoutside with a metallic solution, preferably a resinate solution of two or more noble metals. For the purpose of .securing metal films of low temperature coeicient, it is preferred to use la resinate solution .containing platinum and 4palladium in atomic ratio of 1.5 to 1 respectively. For higher resistance it may be preferable to use an atomic ratio .of 1 to 1. A small amount of rhodium is included for better .rluxing and adherence to the glass tube in baking.

After the tube .has .been coated with the metal solution, it is rotated about its axis while the film is allowed to dry. It is then placed in .an oven and baked .for about seven minutes at a temperature within the range of about 590 C. to about 67.0 C., .the tube being properly .supported Ato prevent bending. The resulting baked metal lm adheres to the glass tube very strongly and has -a very stable Iresista-nce value which can be controlled .to acertainextent .by .controlling the time of baking. The resistance .of a stable lm increases with increase in time of baking.

After the rst coating and baking operation, the resistance value is measured, and if the desired resistance .cannot be obtained within the limits of the coated tube, one or more additional coating `and .baking operations vare applied until the desired .resistance value .is obtained between pointsspaced from the ends of the tube. Next, the ends of the tubes beyond the two spaced points ,are .either .coated with .a metallic lpaste requiring baking only at lower temperature, or they are coated with the same metal solution, or with a stronger .solution .of la single noble metal such as platinum, to provide low resistance connector rings at each I.end of the tube. In the latter cases the .tube is again baked .at a temperature of approximately 650 C. for about live minutes. In

constructions like that shown in Figure 2, the wall of the glass tube should be rounded on each end and the low resistance terminal rings should extend around the end of the tube wall. More than one coating and baking operation may be required to provide the necessary low resistance of the two terminal collars which are used for soldering the two bullet connectors to the two ends of the tube.

As shown in Figure 2, the two bullet connectors have an outside diameter equal to the outer diameter of the inner conductor and are provided with reduced shanks which extend into the ends of the tube 4. The connectors areV preferably formed to provide a shallow groove at-the base of the shoulder which surrounds the stem. In securing the connectors to the tube 4, each connector in turn is held with the stem in a vertical position and is heated while molten solder is deposited in the ring around the stem. One end of the tube 4 is then telescoped over the stem of the bullet and while the tube is maintained in vertical alignment with the bullet, heat is applied to the bullet and when the solder melts the glass is permitted to settle down under some controlled weight against the shoulder of the bullet. Upon removing the heat the solder solidies and a firm soldered joint is obtained between the shoulder of the bullet and the low resistance terminal ring at the end of the tube.

While the preferred procedure is as indicated above, it is possible to rst apply the low resistance terminal bands or collars to the ends of the glass tube before applying the attenuator film.

'Ihe length of the attenuator may be made any desired value depending on the attenuation required, the frequency variation of attenuation, and the degree of matching desired. The complex characteristic impedance of the line having the lm in series arrangement may be written, because of the small value of the thickness,

are resistive and reactive components of impedance, respectively. The attenuation per unit length in nepers/cm. is given by With proper consideration of the reflections arising from the introduction of the film, it is possible to design attenuators having the desired 4 attenuation and match by utilizing the above 1 formulae and resistance measured by direct current. Anticipated performance with respect to frequency variation and eiTects of manufacturing tolerances can also be calculated. For those units Which require low attenuation per wavelength, the match is often sufficiently good without considering effect of length or thickness distribution of the film.

The diameter of tubing into which the element is placed has an effect on the attenuation in that the quantity Zo is dependent on the diameter ratio for coaxial line. This fact can also be utilized to advantage in designing the element.

When the iilmis used as a shunt resistance, We

with Yo the characteristic admittance of the cable without film, and employ a similar pro cedure to that above for design calculations (see Figure 4). Here G and B are the resistive, and reactive parts of admittance, respectively.

In Figure 5 we have shown on an enlarged scale a longitudinal sectional view of a modified form of bullet connector. This construction involves certain advantages over the construction shown in Figure 2 and is the preferred form. The rear end of the shank of the bullet 5 has an outside diameter larger than the `outside diameter of tube 4 which is of the same diameter as the central conductor 2c. The end of the shank is provided with an annular groove 5a into which the end of tube 4 is positioned. Thus a thin metallic collarV 5b having a thickness, a, surrounds the end of tube 4. The shank is of the same diameter as the collar 5b for a distanceb, and then a shallow groove 5c is formed around the surface of the shank and has a width c equal to the width b of the collar. The groove 5c extends as far below the surface of conductor 2c and tube 4 as `the ring 5b extends above these surfaces. The purpose in providing the groove 5c is to compensate for the discontinuity in the conductor shape caused by the collar 5b, in other words, any wave reection caused by collar 5b is compensated or cancelled by a corresponding reflection caused by groove 5c, and vice versa. Any part of the bullet shank lying between the groove 5c vand the end of center conductor 2c has the same outside diameter as the conductor 2c. It will be understood, however, that the end of the conductor 2c may form one side wall of the groove 5c. The tube 4 is provided with the necessary attenuator lm and low resistance terminal rings according to the process described above in connection with Figure 2, and the bullet conductors are soldered to the tube by rst tinning the terminal ring of the tube and then inserting the end of the tube in the annular groove 5a which has previously been filled with solder. This construction provides a stronger mechanical connection of the bullet to the attenuator tube.

Figure 6 is a longitudinal sectional View showing a series attenuator unit embodied in the outer conductor of a coaxial cable or suitable for a wave guide. Inthis construction the attenuating film 4a is formed on the inner surface of an insulating tube 4 which forms a linear section of the outer tubular conductor l of the cable. The center conductor is shown at 2, but this conductor would beomitted in the case of a Wave guide. It will be understood that the film 4a, is formed on the cylinder 4 in the manner described above in connection with Figure 2, the lm being provided with low resistance terminal rings at each end which have suitable contact with the adjacent sections of outer conductor I. For example, the low resistance termina1 rings may be soldered to thinv sleeves formed on the ends of outer conductor l and extending into the cylinder 4 as shown in the drawing.

For the purpose of preventing excessive radiation from the attenuator section of the cable or wave guide, an annular metal shield 'l surrounds 7 the nttenualtor I'cyl'imzl'er il land is provided with parallel end walls Fla land 1b. .For the `LpurLpose o'zf preventing vshield TI trom. .acting as a fshurrt path @around the :attenuator lm, the fend wall sections 1a and 'Ib of shield 'I are not connected directly to outer conductor :I but are connected tothe conductor through sleeves 'lc and 1d a-rranged concentric with conductor I ibut spaced or insulated therefrom. The .space between these two 'sleeves and the conductor I maybe .filled with suitable insulating .material if .desired will be seen .from Figure '6., the outer ends of sleeves Ic and '1d are connected .with conductor "I and these sleeves have an e'iectivellength equal to 1/4 o'f the wavelength to be transmitted. This construction provides an effective .open circuit between the ends -of adjacent sections of tube I and theendwalls of metal shield 1.

Figure 7 'is a 'longitudinal sectional view of a section o'f vcoaxial cable showing one form .of shunt attenuator which may be employed. In this arrangement, the attenuator film Ba .is .formed on one or both -faces of an annular disk 8 formed of suitable insulating material such .as Pyrex glass or quartz. The disk '8 surrounds the inner conductor Zand is positioned within the outer conductor l, and preferably the -film '8a 4is electrically 'connected by lany suitable .means with thesetWo-conductors.

Figure /8 is a transverse sectional view ofa coaxial cable showing three further 4modifications of 'shunt attenuator units which may be employed, and Figure 9 is a longitudinal sectional view of Figure 8 taken along the cutting 'line 9 9. 4One form of shunt 4attenuator unit shown in Figures 8 and 9 comprises a flat plate-'like carrier 9 secured to the inner wall of the outer tubular conductor I and provided with tapering end portions '9a and '9b. These end portions have a length `equal to one-half the Wavelength of transmission. `One or both faces of the carrier 9 are provided with a thin metallic 'nlm formed as described above, .and the film or films are connected, as by soldering, with the innersurface Aof conductor l.

Instead of being .attached 'to the outer conductor, the 'attenuator unit like that shown at 9 may 'be attached to the surface 'of the inner .conductor 2l as shown at "9.

The attenuator unit shown at .IB comprises a plate-like carrier provided with a thin metallic nlm 4on lone or both faces 'thereof and mounted so that the films .extend from the outerconductor I "to the inner Aconductor 2. 'The 'ends of the carrierjl'o 'may 'be 'formed as shown in Eigure I9 or in other known ways to cut down reflection losses.

It will be understood that the .form of attenuator 'shown at 9 in Figure 8 is useful in a wave guide as well as in a coaxial cable.

Figure l 'is a transverse 4sectional view of a rectangular wave guide vIl showing a shunt .attenuator unit I2 mounted therein. This attenuator unit may be formed of a plate-like dielectric carrier I2 having one or both faces thereof provided With a thin metallic coating as described above. The ,plate i2 is mounted parallel with the axis ofthe guide Il and transversely of the narrow dimension of the guide. The plate .may be fixed in position .and the films soldered to the inner walls .of the wave guide, or the attenuator unit may be mounted to be adjusted transversely of the waveguide inforder to place -theiattenuator unit :in iregions of different eld fstre'ngth. It '.is known that vin most .practical rcases the :electro- 8 staitic field is istrongest lat fthe center Irof the guide andl Aecreasesft'owards thelside's. Accordingly, 'by fg the '.pos'itionlef the attenuatoi unit, fas by slfrifting the -unit by means :of 1an insulating lrod 13 extending '-throug'h'one side wall, it is possible to vary the amount yof lattenuation produced :by the unit. No .special provision .need be I'made for insuring contact 4between the-attenuator lmsand the inner walls of the Wave guide, but Iif desired; resilient -eonducting strips can `be inserted ibetween the edges Iof the attenuator unit and #the contacting lw-alls of the 1-wave-1'guide,1the films being 'connected tothe walls through "the l'r'esilierlit strips.

We Jclaim:

l. An attenuator 'unit for alternating cur-rent having substantially constant rate of power abso'rption for -all frequencies below a predetermined frequency, s'a-id unit comprising a dielectric carrier provided with a layer `of :metallic particles formed of a noble metal, said layer yof metallic particles 'having ya thickness of only va small'frac'- Yticn'o'f 'the depth-of penetration of current 'at said predetermined frequency.

2. A-fconductor for high frequency current com'- pris'ing a metal I'lm Yon va dielectric carrier, ysaid film-'comprising the bakedresidue o'f a film of a mixtureof solutions of two'n'oble metals comprising platinum and palladium 'in 4atomic percentages 'of 60 to 50 percent of platinum and 40 to percent of palladium, said residue `film having a lower temperature `coefficient vof resistance vand higher yspecific resistance lthan "either o'f 'the noble Ymetal components.

3. A conductor according to claim 2 wherein said metal film has a thickness less than one-half the depth of penetration for a vfrequency of 300'() megacycles per second.

4. A 'conductor for high frequency currents comprising a sintered'la'y'er of metallic particles deposited 'from a dried mixture of two metallic solutions comprising platinum and palladium 'in atomic percentages of 60 .to peroent of platinum and 40 to 50 percent of palladium.

5. A 'conductor for high frequency currents comprising a uniform mixture of cohering metal particles consisting of platinum and palladium in atomic percentages of 60 to 50 percent platinum and 40 to 50 .percent of palladium.

6. An attenu'ator unit comprising acylindrical dielectric carrier having low resistance terminal collars formed at each end thereof and comprising rings of thin metallic'film, a thin metallic nlm of relatively high resistance on said carrier and bridging said collars, and a connector mounted on each end of said carrier and vhaving good electrical contact with said collars, each connector .comprising a section vof metallic rod of the same outside diameter as said carrier and arranged to abut one end of said carrier, and a soldered connection between each connector and the adjacent terminal collar.

7. An lattenuator unit according to claim 6 wherein said carrier is formed of a tube and said connectors are provided with shank extensions of reduced diameter extending into the ends of said tube.

8. A conductor for ultra-high frequency currents comprising a dielectric carrier having a metal nlm formed thereon-of a thickness of the order-of 1GO angstrom units, said film 'comprising the baked residue of .a film of a'mixture of 'solutions of two alloyable noble metals, said residue film having a lower temperature coefficient vof resistance and .higher specific 'resistance than either of the .two noble metalfco'mponents.

9. A high resistance conductor comprising a thin metallic lm having a thickness of the order of 100 angstrom units and being formed of a uniform mixture of cohering metal particles consisting of platinum and palladium in atomic percentages of 60 to 50 percent of platinum and 40 to 50 percent of palladium.

10. An attenuator unit comprising a cylindrical dielectric carrier having low resistance terminal collars formed at each end thereof and comprising rings of metallic illm bonded directly to the surface of said carrier, and a thin metallic film of relatively high resistance bonded to the surface of said carrier between said collars and having end portions bonded to the outer surfaces of said terminal collars, said high resistance film comprising the baked residue of a lm of a mixture of solutions of two alloyable noble metals, and said residue iilm having a lower temperature coefficient of resistance and higher specific resistance than either of the noble metal components.

11. 'I'he process of constructing a metallic film on a non-conductive carrier, comprising providing a mixture of organo-compound resinates of platinum and palladium, coating the non-conductive carrier with the mixture, drying the coated carrier and thereafter heating same to decompose the organoresinates and precipitate the metals therefrom as an alloy upon the non-conductive carrier and to remove any carbonaceous residue remaining after said precipitation.

Cil

12. An article of manufacture comprising a resistance unit having a substantially constant rate of power absorption at all frequencies below a predetermined frequency, said unit comprising a non-conductive carrier, an alloy lm of at least two substantially stable and non-oxidizable metals tightly bonded to said carrier and having a thickness of less than one-half the depth of penetration at said predetermined frequency, and electrical terminal means for said alloy l'ilm.

References Cited in the file of this patent UNITED STATES PATENTS Number Name vDate 1,296,938 Fahrenwald Mar. 11, 1919 1,905,353 Potter Apr. 25, 1933 1,957,538 Jensen May 8, 1934 2,262,134 Brown Nov. 11, 1941 2,399,645 Latimer May 7, 1946 2,429,401 Davis Oct. 21, 1947 2,437,067 Bingley Mar. 2, 1948 2,440,691 Jira May 4, 1948 FOREIGN PATENTS Number Country Date 486,639 Great Britain June 8, 1938 887,597 France Nov. 17, 1943 

